1. Field of the Invention
This invention relates to a high voltage gallium nitride based semiconductor device and a manufacturing method of the semiconductor device.
2. Description of the Related Art
A gallium nitride (GaN) based semiconductor is widely used all over the world as a material for an optical device represented by a light emitting diode (LED) and a laser diode (LD). Furthermore, the gallium nitride based semiconductor has extremely excellent physical properties of a material such as a wide band gap, a high electron mobility, a high saturation electron speed and a high breakdown voltage, thus in recent years, it is attracting attention also as a material of an electron material represented by a transistor, in particular, its application to a high voltage power device or a high frequency power device is expected.
A metal organic vapor epitaxy (MOVPE) method is capable of precisely control a layer thickness and composition at the time of epitaxially growing a semiconductor layer, and capable of epitaxially growing a semiconductor layer on each surface of a plurality of substrates simultaneously, thus conventionally, the MOVPE method is widely adopted at the time of manufacturing of not only the gallium nitride based semiconductor device, but also a gallium arsenide based semiconductor device such as an aluminum gallium arsenide (AlGaAs) semiconductor device, an indium gallium arsenide (InGaAs) semiconductor device, an indium phosphide based semiconductor device such as an indium gallium phosphide (InGaP) semiconductor device, and other III-V group compound semiconductor device.
A gallium nitride based semiconductor device having a vertical structure configured to supply an electric current in the layer thickness direction of a gallium nitride based semiconductor layer is manufactured by epitaxially growing the gallium nitride based semiconductor layer on a surface of an n-type gallium nitride freestanding substrate by using the above-mentioned metal organic vapor epitaxy (MOVPE) method or the like. In the gallium nitride based semiconductor, mobility of electrons is extremely higher than mobility of positive holes, thus when a high voltage power device is manufactured by using the gallium nitride based semiconductor, it is considered to design such that a carrier concentration of the n-type gallium nitride based semiconductor layer is suppressed to be low and a depletion layer is expanded to the n-type gallium nitride based semiconductor layer when a reverse bias voltage is applied.
If an electric field distribution or a layer thickness of the depletion layer in a Schottky junction or a p-n junction is calculated on the basis of a Poisson equation, it is known that an electric field intensity becomes maximum in the junction surface, the depletion layer is increase in the layer thickness when a high voltage is applied as a reverse bias voltage or the carrier concentration of the n-type gallium nitride based semiconductor layer is suppressed to be low.
Consequently, it is basically preferable that the carrier concentration of the n-type gallium nitride based semiconductor layer is suppressed to be low so that the electric field intensity in the junction surface does not exceed a breakdown voltage, and simultaneously the layer thickness thereof is configured to be thicker than that of the depletion layer, but if the above-mentioned configuration is adopted, an on-resistance is increased when a forward bias voltage is applied.
Therefore, it is considered to suppress the carrier concentration of the n-type gallium nitride based semiconductor layer to be low at an area close to the junction surface and to keep the carrier concentration to be high at an area far from the junction surface for the purpose of reduce the electric field intensity in the junction surface, instead of suppressing the carrier concentration of the n-type gallium nitride based semiconductor layer to be uniformly low over the entire layer. This can be easily considered by solving the Poisson equation to which an appropriate boundary condition is given.
As a donor impurity configured to control an n-type conductivity of the gallium nitride based semiconductor layer, usually silicon (Si), germanium (Ge) or the like is used.
In addition, it is known that carbon (C) included in the gallium nitride based semiconductor layer reduces an electron concentration by compensation effect (e.g., refer to Seager et al., Role of carbon in GaN, Journal of Applied Physics, Vol. 92, No. 11, p. 6553-6560, Dec. 1, 2002). The above-mentioned carbon is not added thereto by intentionally supplying a carbon raw material at the time of epitaxially growing the gallium nitride based semiconductor layer, but is mixed thereinto via a gallium raw material or the like having a carbon-hydrogen (C—H) bond.
It is possible to control a carbon concentration of the gallium nitride based semiconductor layer, for example, if a growth pressure is heightened, the carbon concentration of the gallium nitride based semiconductor layer can be reduced (for example, refer to Seager et al., Role of Carbon in GaN, Journal of Applied Physics, Vol. 92, No. 11, p. 6553-6560, Dec. 1, 2002), and a growth speed is lowered, or a ratio of V/III that means a supply mole ratio between ammonia (NH3) or the like which is a raw material for nitrogen (N) of V group element, and trimethylgallium ((CH3)3Ga) or the like which is a raw material for gallium (Ga) of III group element is increased, the carbon concentration of the gallium nitride based semiconductor layer can be reduced (e.g., refer to Matsumoto et al., High Growth Rate Metal Organic Vapor Phase Epitaxy GaN, Journal of Crystal Growth, Vol. 310, p. 3950-3952, Aug. 15, 2008, and Ubukata et al., High-Growth-Rate AlGaN Buffer Layers and Atmospheric Pressure Growth of Low-Carbon GaN for AlGaN/GaN HEMT on The 6-in.-Diameter Si Substrate Metal-Organic Vapor Phase Epitaxy System, Journal of Crystal Growth, Vol. 370, p. 269-272, May 1, 2013). In addition to the above-mentioned cases, the carbon concentration of the gallium nitride based semiconductor layer can be also drastically changed by changing a kind of a gallium based organic metal material used as a raw material for gallium (e.g., by changing trimethylgallium or triethylgallium ((C2H5)3Ga)).
From a different viewpoint, the relationship between an off-angle of the n-type gallium nitride freestanding substrate and the carbon concentration of the n-type gallium nitride based semiconductor layer has been investigated in detail, it is known that the carbon concentration of the n-type gallium nitride based semiconductor layer can be reduced by using the n-type gallium nitride freestanding substrate having the off-angle of not less than a certain amount (e.g., refer to JP-A-2007-299793). In particular, in case of the high voltage power device, it is important to precisely control a carrier concentration of the n-type gallium nitride based semiconductor layer of which carrier concentration is low, thus it is preferable that the carbon concentration is as low as possible in terms of obtaining a desired electron concentration in a region where a donor impurity concentration is low.